Not all AVRs are created equally... that is why there is a night and day difference for some and not others. A cheap best buy receiver is obviously cheaper for a reason. Transformers. (insert lazer sounds)

It has to do with the current supply rating of the secondary of the power supply transformer. Voltage is one thing, and does provide SPL. Current is also important as it provides depth, punch and clarity. Dynamics if you will. Even at low volume.

A simple way to think of it is a motor's ability to drive a mechanical load with less current vs rated current. If voltage remains constant and the load is increased current must also increase to stop the process from stalling out. If the current is not on hand, ie. the transformer is limited, then the rate of doing work will slow until the process halts. In a speaker the mechanical load is the driver itself. It's ability to quickly change motion and counter linear forces are directly related to current. Now we can understand why headroom is important.

This is why higher end power amplifier separates quote the kVa rating of their transformers. The higher the kVa rating of a transformer, the more current it can theoretically supply if voltage and frequency remains constant. Ie, the rate of work possible is much better.

This is also why subwoofers have dedicated amplifiers. It should stand to reason speakers would benefit from the same ample power treatment. Deciding what is ample is completely subjective. But it can be argued more real power is definitely a good thing, and therefore costs a premium.

HC, Ohm's Law hasn't been repealed, and if it ever is, it'll be revealed in my AES Journal, not on an audio forum, and will probably result in a Nobel Prize in Physics.

Speakers vary widely in impedance with frequency, and at a given frequency and impedance, with a given amount of voltage from the program source, after amplification, the current is specified by Ohm's Law as I=E/R(current equals voltage divided by resistance). Whether the amplifier cost $100 or $100,000, the current has to be the same, and is specified by Dr. Ohm, not by manufacturer hype intended to entice the uninformed or misinformed.

I agree totally. But ohm's law is really only pertinent for static loads purely resistive in nature (heaters/incandescent lights etc.) Once you start to account for magnetic fields, variable frequency, net reactance, efficiency(power factor), linear torque, counter torque, inrush, etc. you have to start looking at motor/transformer theory.

Stricktly speaking I=E/R is not used outside of AC\DC purely resistive circiut calculations... The "I" in this case is the "in phase" current relating to the resistive components in a circiut only. It is not what the load would draw from the source.

Resistance and impedance are not to be confused. Resistance is fixed and does not vary with input voltage or frequency. Reactance is affected by the frequency of the applied voltage, and the net inductive\capacative reactance component (leading or lagging Ohms.) Impedance is then calculated by adding resistance to reactance using trig. Since speaker drivers are a combination reactive\resistive load they require more apparent power (va) to operate than what is acutally used to drive the load mechanically (true power--W)

Speaker drivers can be seen as a motor that has a constantly changing mechanical load. The inertia created by the driver must be overcome, quickly, by a sudden inrush of current to realign magnetic fields and change the driver's motion. Inrush in a circuit usually results in a brief voltage sag-- especially when a transformer is involved (and exhibits poor voltage regulation.) This effect can be observed in your house as your lights dim slightly for a second when the microwave starts etc. Car audio is a good example of this. Large capacitors are often used to mitigate the issue.

A good amplifier is able to compensate for these demands by employing an ample transformer, with good voltage regulation and secondary current rating. They will then usually pair the transformer with a capacitor bank to help with maintaining output voltage. This all relates back to Lenz, Faraday, Henry and others that followed Ohm.

The difference and what everyone seems to ignore is that the capacity of the transformer to provide the current (VA rating) is of high importance-- and often cost.

Ah this brings back memories. Of course Hell is correct, speakers present an inductive reactance so we shouldn't think in terms of ohms but impedance for speakers. In addition in AC circuits with reactive loads we should not think in terms of watts but VA (Volt Amps) or apparent power.

I remember many, many, many moons ago when I had a teacher make us do a lab where this little capacitor was apparently sinking something like 100 watts. The class all came up with the answer and the the prof laughed and opened our eyes to impedance.

_________________________
For every expert, there is an equal and opposite expert.

When a relatively simple and basic point is made briefly, there's some danger of it being inundated with a flood of terminology in response. Of course there was no suggestion that a speaker is a fixed resistor, and the varying impedance(due to capacitive and inductive reactance)with frequency was mentioned. Ohm's Law(sometimes phrased in the I=E/Z form to use Z to emphasize impedance rather than pure resistance)specifies the current in effect in a given speaker at a given frequency and voltage input. This is a fixed amount of current, dependent on the speaker, which can be neither lower nor higher, regardless of the amplifier.

This was pointed out simply because of an incorrect implication in a previous reply that the capabilities of an amplifier could change the current specified by Ohm's Law and that somehow the speaker would then work better.

Ohm's Law(sometimes phrased in the I=E/Z form to use Z to emphasize impedance rather than pure resistance)

John, i've never seen Ohm's law used in any AC circuite analysis.. Usually for AC circuit analyses the minimum math required is algebra, and more likely deferential calculus, due to the vector calculations required.. there are constantly changing variables of the inductance and capacitance within the circuit, Ohm's law is not sophisticated enough to do the calculations.

Here is a quick example of the differences that i am talking about, notice all of the resistances have vectors associated with them. Ohm's law has no way at all of taking angle vectors into account. it is to simple of an eqn intended to be used for a static DC circuit.